1. Field of the Invention
The invention is related to an active solid-state device. In particular, the invention relates to a bi-directional transient voltage suppression device and forming method thereof.
2. Description of the Prior Art
Transient voltage suppression (TVS) devices comprising a zener diode are well known. As the reverse avalanche voltage becomes smaller, a depletion region of the zener diode narrows and the zener diode has a higher internal capacitance. Especially, when the operating frequency becomes higher, the higher internal capacitance of the zener diode becomes a serious issue for the device. A well known solution to this problem is to add a rectifier diode in series with the zener diode. Either the anodes or the cathodes of the zener diode and the rectifier diode are coupled together. The capacitance of the rectifier diode is smaller than that of the zener diode, and the total capacitance of the zener diode and the rectifier diode in series is less than the sum of the two capacitances.
A transient voltage suppression device comprising such a pair of diodes is a unidirectional transient voltage suppression device and provides protection against voltage spikes or surges for I/O signal with positive signal only. In addition, a bi-directional transient voltage suppression device comprising two such pairs of diodes in an anti-parallel configuration is also known. The reverse avalanche voltage, or the breakdown voltage, is defined as the voltage at which the zener diode goes into avalanche mode, measured at a relatively low current such as one milliampere. The breakdown voltage is controlled by the doping level of an N type diffusion layer relative to the doping level of a P type diffusion layer of the zener diode.
The clamping voltage is defined as the maximum voltage across the transient voltage suppression device when a maximum surge current is flowing through it. The clamping voltage is typically measured at a relatively high current such as one ampere. As a result, the clamping voltage is normally higher than the breakdown voltage. The clamping voltage of transient voltage suppression device is proportional to the breakdown voltage of the zener diode. The amount by which the clamping voltage is greater than the breakdown voltage is directly proportional to the geometry of the PN junction and to the diffusion depth of the zener diode.
As the size of an electronic device (i.e. a mobile phone) becomes smaller, there is a need for a smaller transient voltage suppression device. However, in prior arts, the clamping voltage of transient voltage suppression device will disadvantageously rise if the zener diode was simply made smaller because a smaller PN junction area. This is because that the smaller PN junction area has a higher resistance.
Please refer to the patent U.S. Pat. No. 7,361,942, U.S. Pat. No. 7,361,942 discloses a transient voltage suppression device including a semiconductor die that has a first avalanche diode in series with a first rectifier diode connected cathode to cathode, electrically coupled in an anti-parallel configuration with a second avalanche diode in series with a second rectifier diode also connected cathode to cathode. However, it can be found that the device of U.S. Pat. No. 7,361,942 is actually a uni-directional device only, so that the device of U.S. Pat. No. 7,361,942 can not protect the IO signal with positive and negative.
Therefore, the main goal of the invention is to provide a bi-directional transient voltage suppression device and forming method thereof to solve the problems above.